This invention relates to an apparatus for using a Coriolis mass flowmeter with a serial, dual loop, flow tube for measuring the flow rate of a fluid through a pipeline. More particularly, the invention relates to the element used to connect the loops of the flow tube. Still more particularly, the invention relates to an anchor which connects a flow tube to a flow tube housing.
It is known to use Coriolis effect mass flowmeters to measure mass flow and other information of materials flowing through a pipeline as disclosed in U.S. Pat. No. 4,491,025 issued to J. E. Smith, et al. of Jan. 1, 1985 and U.S. Pat. No. Re. 31,450 to J. E. Smith of Feb. 11, 1982. These flowmeters have one or more flow tubes of a curved configuration. Each flow tube configuration in a Coriolis mass flowmeter has a set of natural vibration modes, which may be of a simple bending torsional, or coupled type. Each flow tube is driven to oscillate at resonance in one of these natural modes. The natural vibration modes of the vibrating, material filled system are defined in part by the combined mass of the flow tubes and the material within the flow tubes. Material flows into the flowmeter from a connected pipeline on the inlet side of the flowmeter. The material is then directed through the flow tube or flow tubes and exits the flowmeter to a pipeline connected on the outlet side.
A driver applies force to oscillate the flow tube. When there is no flow through the flowmeter, all points along a flow tube oscillate with an identical phase. As the material begins to flow, Coriolis accelerations cause each point along the flow tube to have a different phase with respect to other points along the flow tube. The phase on the inlet side of the flow tube lags the driver, while the phase on the outlet side leads the driver. Pickoff sensors are placed on the flow tube to produce sinusoidal signals representative of the motion of the flow tube. The phase difference between the two pick off sensor signals is proportional to the mass flow rate of the material flowing through the flow tube or flow tubes.
Material flow though a flow tube creates only a slight phase difference on the order of several degrees between the inlet and outlet ends of an oscillating flow tube. When expressed in terms of a time difference measurement, the phase difference induced by material flow is on the order of tens of microseconds down to nanoseconds. Typically, a commercial flow rate measurement should have an error of less the 1%. Therefore, a Coriolis flowmeter must be uniquely designed to accurately measure these slight phase differences.
It is known to use a single loop, serial path flow tube to measure the rate of fluid flowing through a pipeline. However, the single loop, serial flow tube design has a disadvantage in that it is inherently unbalanced. A single loop, serial flow Coriolis flowmeter has a single curved tube or loop extending in cantilever fashion from a solid mount. Dual loop Coriolis flowmeters are balanced. A dual loop Coriolis flowmeter has two parallel, curved tubes or loops extending from a solid mount. The parallel flow tubes are driven to oscillate in opposition to one another with the vibrating force of one flow tube canceling out the vibration force of the other flow tube. The result is that in a properly constructed dual loop Coriolis flowmeter there are no flowmeter induced vibrations at the points of attachment between the flowmeter and the pipeline. This is called a xe2x80x9cbalancedxe2x80x9d flowmeter. The absence of vibrations allows dual looped Coriolis flowmeter to be attached free standing to a pipeline. A single loop, serial path Coriolis flowmeter must be secured firmly to a support against which the flow tube can vibrate. The use of a support renders the use of a single loop, serial flow tube design impractical in most industrial applications because the serial flow tube requires that the pipeline be near an object that could be used as a support. Therefore, the dual loop flowmeter designs are desirable.
It is a particular problem to measure minimal flow rates of materials flowing through a pipeline. A mass flow rate through a pipeline of less than or substantially equal to 4 lbs. per minute is considered minimal for commercial applications. A Coriolis mass flowmeter measuring such small flow rates must be formed of relatively small components including tubes and manifolds. These relatively small components present a variety of challenges in the manufacturing process including but not limited to difficult welding processes.
One solution for measuring minimal flow rates has been to use a single loop, serial flow tube Coriolis effect mass flowmeter. Single loop, serial flow tube Coriolis flowmeters have certain advantages. The flow tube has a larger diameter which reduces pressure drop across the flowmeter. No manifold is necessary to split the flow into two tubes. The larger flow tube is easier to draw and weld. There are also other advantages. The problem is that single loop, serial flow tube flowmeters cannot be mounted free standing into the pipeline since they are not balanced flowmeters.
Dual loop, parallel flow tube flowmeters can be mounted freestanding into the pipeline. However, the small size necessary for measuring minimal flow rates creates design and manufacture problems for use of the dual loop, parallel flow tube design. These problems limit the industrial applications of dual loop, dual tube Coriolis flowmeters for measuring minimal flow rates.
A particular problem with dual loop, parallel flow tube design is that a manifold must be used to direct the flow entering the inlet end of the flowmeter in order to divide the flow so that it enters the two flow tubes. It is difficult to produce a manifold, by casting or otherwise, in the small dimensions necessary to measure a minimal flow rate. Also, the manifold increases pressure drop across the flowmeter. Further, the flow tubes must be welded or brazed onto the manifold. It is difficult too weld very thin walled tubing. The welds and joints do not provide the smooth surface needed for sanitary applications of the flowmeter. Sanitary applications demand a continuous, smooth flow tube surface that does not promote adhesion of material to the walls of the flow tube. Further, the additional welds there are necessary reduce the manufacturing yield. Therefore, the use of a manifold is not desired in flowmeters designed for measuring minimal flow rates.
The smaller diameters of the dual flow tubes make the tubes more prone to plugging. The smaller diameter is needed to assure a sufficient flow rate through the flow tubes. Material is more likely to plug the flow path through these flow tubes because smaller particles in the material can obstruct the smaller flow path. These obstructions can cause inaccurate readings of the flow rate and breakage of the flow tube. Therefore, the dual flow tube design does not offer a satisfactory solution for measuring minimal flow rates.
A further problem is that sometimes a Coriolis flowmeter is used to measure flow through a pipeline where the flowing material is pressurized. If a flow tube cracks, the pressured material will rapidly spray from the highly pressurized flow tube to the outside surroundings which have a lower pressure than the flow tube. The pressurized material spraying from the flow tube can damage the pipeline or surrounding structures.
The above and other problems are solved by the apparatus of the present invention that comprises a dual loop, serial path flow tube. Each of the loops is oriented in a plane parallel to the plane containing the other loop. The flow tube is enclosed in a housing to which the flow tube is connected through an anchor. The housing can be configured to contain the leakage of pressurized materials from a break in the flow tube. These advantages allow the present invention to be used to measure the flow rates, including minimal flow rates, of material flowing through the pipeline.
In the present invention, the dual loops in the serial flow tube are connected by a crossover section. The outlet end of the first loop connects to an inlet end of the crossover section in the plane containing the first loop. The inlet end of the second loop connects to an outlet end of the crossover section in the plane of the second loop. The crossover section of the flow tube allows the present invention to have the advantages of both serial and parallel flow tube designs for measuring minimal flow rates.
The present invention has a serial flow tube. Serial flow tube and parallel flow tube flowmeters each have advantages and disadvantages. For the same tube parameters, i.e. inside tube diameter, tube wall thickness, and tube geometry, an oscillating serial flow tube generates more Coriolis force than an oscillating parallel flow tube since all the flow passes through each portion of a serial flow tube instead of only half of the flow passing through each portion of a parallel flow tube.
The disadvantage of a serial flow tube is that the pressure drop through a serial flow tube is higher than for a parallel flow tube with the same tube parameters. To reduce pressure drop, a sensor with a serial flow tube typically uses a larger diameter and proportionally thicker flow tube wall to achieve substantially the same pressure drop of a parallel flow tube flowmeter. Therefore, serial path Coriolis flowmeters are inherently larger than parallel path flowmeters. Generally this is a disadvantage for Coriolis flowmeters. However, for minimal flow rate sensors it is an advantage. A flow tube with a greater diameter reduces the probability of particles plugging the flow tube. The joining, by welding or brazing, of a relatively larger diameter, heavier wall flow tube make the flowmeter design of the present invention easier to produce and better suited for sanitary applications. Therefore the flowmeter of the present invention can be used for industrial applications in which a typical dual loop, parallel flow flowmeter cannot be used.
The present invention is also an improvement over the dual loop, parallel flow tube flowmeters because the present invention does not need a manifold. Manifolds are needed in dual flow tube designs to divide the flow entering the flowmeter into the two flow tubes. Since the present invention has a serial flow tube, a manifold is not needed to divide the flow. Thus, the flow tube of the present invention is easier to weld as there are fewer welds.
The two loops of the flow tube of the present invention are oscillated in opposition to one another. Vibrations caused by the oscillation of the loops are canceled out and do not affect the ends of the flowmeter. Therefore, the flowmeter of the present invention is balanced and does not have to be attached to a support. Thus, the flowmeter of the present invention may be attached freestanding in a pipeline without mounting the flowmeter to a support.
The flow tube of the present invention is secured, near the crossover section, by an anchor. The anchor is the solid mounting from which the dual loops of the flow tube extend in cantilever fashion. The anchor is fixed to a flowmeter housing. The inlet and outlet of the flow tube are connected to the housing through an adapter which transitions the fluid from the flow tube to a process connection. The process connections are flanges or the like for connecting the flow tube to the process pipeline. Therefore the flow tube, anchor, and housing share a common physical reference. The housing can be designed to contain the leakage of a pressurized fluid in the case breakage of the flow tube. The anchor connected to the housing and flow tube holds the flow tube securely in place with enough room to oscillate freely inside the housing. The anchor is used to attach the flow tube to the housing to minimize the effect of distortions of the flow tube that would be caused if the flow tube were attached directly to the housing with welds. Also the anchor decouples the vibrating portion of the flowmeter, above the anchor, from the non-vibrating portion of the flowmeter where the flowmeter attaches to the pipeline.
The inlet and outlet portions of the flow tube of the present invention can be formed to any desirable configuration. For example the inlet and outlet portions of the flow tube can be formed in-line with each other or the meter can be made self-draining by forming them in a spiral, off-set configuration.
The modular design of the flowmeter of the present invention makes it relatively easy for the designer to make changes to the wetted components of the flow tube. Since the fluid only contacts the flow tube and the adapters, the housing and anchor can be used with flow tubes and adapters of different materials without necessarily making any further design changes.
The apparatus of the present invention has the above described and other advantages in measuring the flow rate of material flowing through pipelines. Unlike traditional Coriolis flowmeters, the present invention has a serial, balanced flow tube. A crossover section in the flow tube connects two loops in the flow tube. The configuration of the serial flow tube allows the present invention to behave like a dual flow tube flowmeter, while having serial flow tube characteristics. The anchor and housing configuration provide support for the flow tube and minimize distortion of the flow tube.